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Instead of using a motor encoder, what electronics sensor I could use to know the position of a satellite dish? In this case will be the Azimuth and Altitude of the dish (2-axis).

Magnetometer, Accelerometer or Gyro? any particular model I could get in eBay or Sparkfun?

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    \$\begingroup\$ Any reason you don't want to use a rotary encoder? It would strike me as much more accurate and have no drift compared to the other options you've listed. It wouldn't necessarily have to go on the motor shaft if that has some sort of clutch arrangement between that and the dish. \$\endgroup\$
    – PeterJ
    Jul 25, 2013 at 13:01
  • \$\begingroup\$ @PeterJ if using a rotary encoder, for this application what kind of rotary resolution would I need? 3600 steps/revolution for 0.1 degree percision? \$\endgroup\$
    – Dennis
    Jul 25, 2013 at 14:14
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    \$\begingroup\$ Yes and they are really expensive although you could probably use a gearbox as a cheap solution combined with a lower resolution one but you'd need to be aware of backlash. A gyro wouldn't help (that's change of rotation) and I doubt a MEMS accelerometer would get close to 0.1 degrees especially if it happens to be pointing pretty well upwards. \$\endgroup\$
    – PeterJ
    Jul 25, 2013 at 14:31

4 Answers 4

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A stationary object may use an accelerometer to reasonably accurately determine which direction is down (or, conversely, up) without needing any sort of reference other than a large nearby planet. In theory, a sufficiently-precise gyroscope could also determine which directions were north and east without needing any sort of reference other than being affixed to a large nearby planet which is turning on a north-south axis, but I doubt that any commercially-available electronic gyroscopes would achieve that level of precision.

If you have a base whose alignment is fixed, and merely wish to measure position relative to that, potentiometers can be pretty good. When they are used in a ratiometric fashion, they can be relatively precise and also immune to changes in temperature, humidity, etc.

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    \$\begingroup\$ If that planet just happens to have a magnetic field, a 3 axis magnetometer can provide orientation information as well. The accelerometers can be used to determine the magnetometer's orientaion vs. vertical to aid in the calculation. Has it's limitations, but would be less complex than gyrocommpassing. \$\endgroup\$
    – B Pete
    Jul 25, 2013 at 16:54
  • \$\begingroup\$ @BPete: A magnetic compass may be usable in some places, but I'm not sure how much accuracy one should assume in the proximity of equipment that might be made of steel. Someone in a cave with sufficiently-precise gyros could, given time, ascertain the direction of the Earth's axis even if the gyros were initially uncalibrated. \$\endgroup\$
    – supercat
    Jul 25, 2013 at 17:07
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why not try a rotary potentiometer.

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  • \$\begingroup\$ what kind of rotary potentiometer precision I would need to use for my case? and how accurate does a rotary potentiometer when subject to temperature changes? \$\endgroup\$
    – Dennis
    Jul 25, 2013 at 14:17
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If this is a DIY project you might want to use an optical encoder like Agilent AEDR-8300.

enter image description here

It is the same sensor principle which is used inside of a relative rotary encoder, but you can print the reflector stripe yourself. This is just a white/black 2bit gray code. So you can create much better resolutions without a gearbox. No drift, no worries. Okay, maybe sunlight, rain, ... but I think the optical way is fairly robust.

edit: You don't even have to print a gray code pattern. A black/white stripe is sufficient. See the datasheet.

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An RVDT (Radial Differential Voltage transformer) might offer a better resolution than a rotary encoder, however it requires a more complex circuitry.

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